It is well known in the art to use a fan to provide cooling to an internal combustion engine. The fan is typically driven by a belt connected to a pulley on the engine. It is also known to employ a viscous clutch between the fan's pulley, which receives drive power from the engine, and output of the clutch coupled to the fan.
A typical system is shown in FIG. 1. An engine 10 is disposed in a vehicle 12. On the front of engine is a pulley (not visible in FIG. 1) which drives a second pulley 14. Pulley 14 is coupled to a shaft 16. Shaft 16 is coupled to a viscous clutch 18. Fan 20 is affixed to viscous clutch 18.
A front view of engine 10 is shown in FIG. 2 in which pulley 24, which is affixed to the crankshaft of the engine, is coupled to pulley 14 via belt 26. Pulley 16 is coupled to viscous clutch 14, which is in turn coupled to fan 20.
A detail of the viscous clutch 18 is shown in FIG. 3. Input shaft 16 to clutch 18 is coupled to plate 30. Output shaft 22 is coupled to plate 32. When the space between plates 30 and 32 is filled with a liquid, e.g., hydraulic fluid, shaft 22 is caused to rotate at a speed somewhat less than the rotational speed of shaft 16, such speed of shaft 22 depending on speed of shaft 16, viscosity of the fluid and other parameters. When the space contains little or no liquid, shaft 22 rotates at a much slower speed than shaft 16. Viscous clutch 18 contains a storage reservoir 28 containing a hydraulic fluid. The space in between plates 30 and 32 is called a drive chamber. There is a valve (not shown) between storage reservoir 28 and the drive chamber. In one example, the valve is an orifice that can be occluded or not by a plate. In a second example, the valve is an orifice that can be occluded by a pin. When the valve is open, hydraulic fluid from the storage reservoir is forced through the valve orifice into the drive chamber, thereby filling the space between plates 30 and 32. Typically, plates 30 and 32 have annular ridges on their surfaces to inhibit the hydraulic fluid movement toward the outside edges and to increase surface area for more torque capacity due to the shear force of the hydraulic fluid. Hydraulic fluid returns to storage chamber 28 through a passageway, not shown. When the valve is open, hydraulic fluid is continually flowing through the circuit described: storage chamber 28, through the valve to the drive chamber (between plates 30 and 32), and back to storage chamber 28 through the passageway.
The opening and closing of the valve is controlled by a bimetallic strip coupled to the valve. One example is a bimetallic coil made with metals of dissimilar expansion coefficients. When the temperature of the coil changes, the coil either coils further or becomes uncoiled, depending on whether the metal with the higher expansion coefficient is on the outside or inside surface of the bimetallic coil. When the bimetallic coil is connected to a small plate that can occlude the valve's orifice, then, the amount of the valve orifice occlusion is a function of how much the bimetallic coil is coiled up, and hence a function of the temperature. In another example, the bimetallic strip is a strip which changes from flat to cupped when it is heated. When the strip becomes heated, it can depress a pin in the valve to cause the valve to open. The bimetallic strip/valve system can be designed to an on-off or fully variable device.
In the prior art, the bimetallic strip is exposed to under-hood air. As the under-hood temperature changes, the bimetallic strip changes shape and changes the position of the valve coupled to the drive chamber and, hence, the amount of fluid in the drive chamber.
In U.S. Pat. No. 4,351,426, a heater is disposed next to the bimetallic strip so that by controlling the current or voltage applied to the heater, the state of the bimetallic strip, and hence the valve opening position, can be controlled. In '426, the bimetallic strip and heating element are enclosed at the back of the viscous clutch. The inventor of the present invention has recognized a disadvantage in such a configuration.